Printing of packaging materials is carried out by gravure printing, offset printing, flexographic printing or the like and gravure printing is frequently used for packaging materials requiring a display effect on goods, because of excellent reproducibility up to fine portions of a design and gradation to produce photograph-like printing. The gravure printing process comprises placing oil ink on concaves on a drum surface engraved by conventional gravure, intaglio gravure, electronic photoengraving or the like, transferring the ink to raw web, and then, blowing hot air to evaporate the solvent of the ink to dryness. The oil ink is a dispersion of pigments in a vehicle produced by dissolving a resin, such as a polyurethane, acrylic resin, nitrocellulose or chlorinated polyolefin, into a solvent.
A conventional solvent is a mixture of toluene (40%)-ethyl acetate (40%)-isopropyl alcohol (20%), methyl ethyl ketone (40%)-ethyl acetate (40%)-isopropyl alcohol (20%) or the like. The solid content, i.e. resin and pigment, of the oil ink is, in common, 8–10%, and in the case of white solid ink requiring a shielding ability, those having a solid content up to 30% are used. In general, the temperature of the hot air is 55–60° C., and blowing volume is 30–70 m3/min. Under these conditions, it is possible to set a printing speed at 120–200 m/min. In the multicolor printing, the number of colors employed is 2 to 10, and the greater the number of colors, the more the print becomes photograph-like decorative.
Packaging, materials frequently used for printing are films of polyethylene terephthalate (PET), oriented polypropylene (OPP), oriented nylon (O-NY), etc., and other applicable films are single layer films of polyethylene (PE), polypropylene (PP) polystyrene (PS), polyvinyl chloride (PVC), etc., shrinkable films of PET, PP, PS, PE, PVC, etc., and stretched films of PE, PVC, etc.
Recently, gravure printing is moving to an aqueous process using an aqueous ink, due to the problems of the solvent in the oil ink, noxious odors on the printing work, adverse effects on the health in the working atmosphere, possibility of explosion, residual solvent odor in prints, contamination of environment around a factory, reduction of CO2 and spending of solvent resources because all the solvent is volatilized in the process of printing (Japanese Patent 3249223, JP 2001-030611A, JP 2002-096448A).
However, the solvent used in the aqueous ink is, e.g., water (70%)-ethanol (30%) having a latent heat of vaporization of 470.7 cal/g, which is great compared with the solvent of oil ink, e.g. 101.9 cal/g for toluene (40%)-ethyl acetate (40%)-isopropyl alcohol (20%) or 109.1 cal/g for methyl ethyl ketone (40%)-ethyl acetate (40%)-isopropyl alcohol (20%). That is, calories required for drying are as much as 4.3–4.6 times that of conventional oil inks. Properties of principal solvents used in gravure ink are summarized in Table 1.
TABLE 1Vap. LatentVaporSurfaceInflammationHeatPressureTensionPoint(b.p.)(20° C.)(25° C.)Solvilityb.p.(closed)(KJ/kg)(Pa)(mN/m)ParameterSolventM.W.(° C.)(° C.)(cal/g)(mmHg)(dyne/cm)(Hansen)Toluene92.1110.64.4363.6400027.98.9186.930.027.9Ethyl Acetate88.176.7−7.2369970623.89.1088.272.823.8Methyl Ethyl72.179.6−4.0439949324.09.27Ketone105.271.224.0Isopropanol60.182.311.7666432021.711.50159.232.421.7*n-propanol60.197.115.0680193323.811.97162.614.523.8*Ethanol46.778.316.0833799922.112.92199.260.022.1Water18.0100.0—2456233371.823.50586.917.571.8*20° C.
A countermeasure is to raise the solids content of the aqueous ink by increasing the quantity of pigments within the range where the concentration of print is not changed, even using a small volume of aqueous ink. This has a limit up to an increase of 20% in the concentration of the oil ink. Thus, even when increasing the quantity of pigment, it is still necessary to supply 3.4–3.7 times the heat in the case of an oil ink, which requires the lengthening of the staying time in the drying process, increasing the blowing volume of the hot air and raising the temperature of the hot air, or the like.
However, lengthening the staying time is undesirable because of lowering the printing speed, and increasing the blowing volume degrades energy efficiency and causes flapping of raw web by the air. In view of heating efficiently, it is most suitable to raise the temperature of the hot air, but it results in raising the temperature of the raw web that includes slippages between printing pitches of each color caused by elongation of the web.
Temperature dependencies of the elongation (pitch elongation) of PET film, O-NY film and OPP film are shown in FIG. 3. The elongation of OPP film is the greatest, followed by O-NY film, and then PET film.
The slippage of printing pitches is rectified by reading color control marks in a form of a trapezoid (almost triangular) printed at an edge of a raw web by a scanning head, and when the distance (20.0 mm) from the trailing end of the base (10 mm) of the trapezoid mark to the front end of the next mark slips with a length of 0.2 mm or more, the length of the passage up to printing is aligned by moving automatically a compensator roll slightly.
However, when the elongation is great, deformation occurs caused by the elongation of the pattern printed in the previous printing unit (deformation of the pattern printed in the first printing unit is the greatest). As a result, color drift (slippage of printing) occurs between the previous print pattern and a pattern printed thereon, and it cannot be eliminated by the compensator roll.
By the way, PET film and O-NY film of which the elongation is small, can be printed at a printing speed of 120 m/min or higher at a temperature of the hot air for drying of 120° C. or higher, but OPP film, of which the elongation is great, cannot be printed due to the deformation of the patterns, which includes color drift (slippage of printing), although OPP film is cheap and widely used.